Diabetes mellitus is an ever-increasing threat to human health. For example, in the United States current estimates maintain that about 16 million people suffer from diabetes mellitus.
Type I diabetes, also known as insulin-dependent diabetes mellitus (IDDM), is caused by the autoimmune destruction of the insulin producing pancreatic β-cells, and necessitates regular administration of exogenous insulin. Without insulin, cells cannot absorb sugar (glucose), which they need to produce energy. Symptoms of Type I diabetes usually start in childhood or young adulthood. People often seek medical help because they are seriously ill from sudden symptoms of high blood sugar (hyperglycemia).
Type II diabetes, also known as non-insulin-dependent diabetes mellitus (NIDDM), manifests with an inability to adequately regulate blood-glucose levels. Type II diabetes may be characterized by a defect in insulin secretion or by insulin resistance, namely those that suffer from Type II diabetes have too little insulin or cannot use insulin effectively. Insulin resistance refers to the inability of body tissues to respond properly to endogenous insulin. Insulin resistance develops because of multiple factors, including genetics, obesity, increasing age, and having high blood sugar over long periods of time. Type II diabetes, sometimes called mature or adult onset diabetes, can develop at any age, but most commonly becomes apparent during adulthood. The incidence of Type II diabetes in children, however, is rising.
In diabetics, glucose levels build up in the blood and urine causing excessive urination, thirst, hunger, and problems with fat and protein metabolism. If left untreated, diabetes mellitus may cause life-threatening complications, including blindness, kidney failure, and heart disease.
Type II diabetes accounts for approximately 90-95% of diabetes cases, killing about 193,000 U.S. residents each year. Type II diabetes is the seventh leading cause of all deaths. In Western societies, Type II diabetes currently affects 6% of the adult population with world-wide frequency expected to grow by 6% per annum. Although there are certain inheritable traits that may predispose particular individuals to developing Type II diabetes, the driving force behind the current increase in incidence of the disease is the increased sedentary life-style, diet, and obesity now prevalent in developed countries. About 80% of diabetics with Type II diabetes are significantly overweight. As noted above, an increasing number of young people are developing the disease. Type II diabetes is now internationally recognized as one of the major threats to human health in the 21st century.
Type II diabetes currently is treated at several levels. A first level of therapy is through the use of diet and/or exercise, either alone or in combination with therapeutic agents. Such agents may include insulin or pharmaceuticals that lower blood glucose levels. About 49% of individuals with Type II diabetes require oral medication(s), about 40% of individuals require insulin injections or a combination of insulin injections and oral medication(s), and about 10% of individuals may use diet and exercise alone.
Current therapies for diabetes mellitus include: insulin; insulin secretagogues, such as sulphonylureas, which increase insulin secretion from pancreatic β-cells; glucose-lowering effectors, such as mefformin which reduce glucose production from the liver; activators of the peroxisome proliferator-activated receptor-γ(PPAR-γ), such as the thiazolidinediones, which enhances insulin action; and α-glucosidase inhibitors which interfere with gut glucose production. There are, however, deficiencies associated with currently available treatments, including hypoglycemic episodes, weight gain, loss in responsiveness to therapy over time, gastrointestinal problems, and edema.
There are several areas at which research is being targeted in order to bring new, more effective, therapies to the marketplace. For example, on-going research includes exploring a reduction in excessive hepatic glucose production, enhancing the pathway by which insulin transmits its signal to the cells such that they take up glucose, enhancing glucose-stimulated insulin secretion from the pancreatic β-cells, and targeting obesity and associated problems with fat metabolism and accumulation.
GIP and GLP-1 are peptides, known as incretins, secreted from enteroendocrine K and L cells respectively in response to ingestion of nutrients, and have a wide variety of physiological effects that have been described in numerous publications over the past two decades. See, for example, Bojanowska, E. et al., Med. Sci. Monit., 2005, August 11(8): RA271-8; Perry, T. et al., Curr. Alzheimer Res., 2005, July 2(3): 377-85; and Meier, J. J. et al., Diabetes Metab. Res. Rev., 2005, March-April; 21(2); 91-117 (each herein incorporated by reference with regard to a background understanding of incretins). Moreover, although the mechanisms regulating GLP-1 secretion remain unclear, the initial rapid rise in GLP-1 following a meal may be a result of hormonal stimulation of neuronal afferents involving GIP. See, for example, J. N. Roberge and P. L. Brubaker, Endocrinology 133 (1993), pp. 233-240 (herein incorporated by reference with regard to such teaching). Furthermore, later increases in GLP-1 may involve direct activation of L-cells by nutrients in the distal small-intestine and the colon. GIP and GLP-1 are potent stimulators of the body's ability to produce insulin in response to elevated levels of blood sugar.
In Type II diabetes the action of GLP-1 on the β-cell is maintained, although GLP-1 secretion, itself, is reduced. More recently, therefore, much research has been focused on GLP-1. Studies show glucose-lowering effects in addition to GLP-1's ability to stimulate glucose-dependent insulin secretion including, but not limited to, an inhibition of the release of the hormone glucagon following meals, a reduction in the rate at which nutrients are absorbed into the bloodstream, and a reduction of food intake. Studies demonstrate that treatments to increase GLP-1, therefore, may be used for a variety of conditions and disorders including but not limited to metabolic disorders, gastrointestinal disorders, inflammatory diseases, psychosomatic, depressive, and neuropsychiatric disease including but not limited to diabetes mellitus (Type 1 and Type 2), metabolic syndrome, obesity, appetite control and satiety, weight loss, stress, inflammation, myocardial ischemia/reperfusion injury, Alzheimer's Disease, and other diseases of the central nervous system. In Type II diabetes, patients display a decreased responsiveness to GIP but not GLP-1, with respect to its ability to stimulate insulin secretion. The mechanism behind the decreased responsiveness to GIP remains unclear since Type II diabetics retain sensitivity to a bolus administration of GIP but not to a continuous infusion (Meier et al. 2004 Diabetes 53 S220-S224). Moreover recent studies with a long-acting fatty-acid derivative of GIP showed beneficial effects on glucose homeostasis in ob/ob mice following 14 days of treatment (Irwin N. et al. (2006) J. Med. Chem. 49, 1047-1054).
The use of exogenous GLP-1 in clinical treatment is severely limited, however, due to its rapid degradation by the protease DPP-IV. There are multiple GLP-1 mimetics in development for type 2 diabetes that are reported in the literature, all are modified peptides, which display longer half-lives than endogenous GLP-1. For example, the product sold under the tradename BYETTA® is the first FDA-approved agent of this new class of medications. These mimetics, however, require injection. An oral medication that is able to elevate GLP-1 secretion is desirable. Orally available inhibitors of DPP-IV, which result in elevation in intact GLP-1, are now available, such as sitagliptin, marketed under the brand name JANUVIA®. Nevertheless, a molecule which may stimulate GLP-1 secretion would provide a therapeutic benefit. A molecule which could stimulate both GLP-1 secretion and insulin secretion through effects on the L-cell and direct effects on the β-cell would hold much promise for Type II diabetes therapy.
One particular target is GPR119. GPR119 is a member of the rhodopsin family of G-protein-coupled receptors. In addition to the “GPR119” identifier, several other identifiers exist, including but not limited to RUP 3, Snorf 25, 19 AJ, GPR116 (believed to be erroneous), AXOR20, and PS1. GPR119 is expressed in human gastrointestinal regions and in human islets. Activation of GPR119 has been demonstrated to stimulate intracellular cAMP and lead to glucose-dependent GLP-1 and insulin secretion. See, T. Soga et al., Biochemical and Biophysical Research Communications 326 (2005) 744-751, herein incorporated by reference with regard to a background understanding of GPR119.
Agonists to GPR119 may be of therapeutic value for diabetes and associated conditions, particularly Type II diabetes, obesity, glucose intolerance, insulin resistance, metabolic syndrome X, hyperlipidemia, hypercholesterolemia, and atherosclerosis.